FOAM RISE

Froth flotation (qv) is a significant use of foam for physical separations. It is used to separate the more precious minerals from the waste rock extracted from mines. This method reHes on the different wetting properties typical for the different extracts. Usually, the waste rock is preferentially wet by water, whereas the more valuable minerals are typically hydrophobic. Thus the mixture of the two powders are immersed in water containing foam promoters. Also added are modifiers which help ensure that the surface of the waste rock is hydrophilic. Upon formation of a foam by bubbling air and by agitation, the waste rock remains in the water while the minerals go to the surface of the bubbles, and are entrapped in the foam. The foam rises, bringing... 

Mechanical Properties and Structural Performance. As a result of the manufacturing process, some cellular plastics have an elongated cell shape and thus exhibit anisotropy in mechanical, thermal, and expansion properties (35,36). Efforts are underway to develop manufacturing techniques that reduce such anisotropy and its effects. In general, higher strengths occur for the paraHel-to-rise direction than in the perpendicular-to-rise orientation. Properties of these materials show variabiUty due to specimen form and position in the bulk material and to uncertainty in the axes with respect to direction of foam rise. Expanded and molded bead products exhibit Httie anisotropy. 

To prepare it, small nuggets of unsalted butter are melted slowly in a deep saucepan. The water in the butter evaporates, causing the milk solids to sink to the bottom of the pan, and a foam rises to the surface and is skimmed off. The clear, yellow melted butter is then poured off to leave the milk solids at the bottom of the saucepan, to be discarded. 

Transfer extract from the suction flask to a 250-ml volumetric flask, rinse the suction flask several times with w (transferring the rinses into the volumetric flask) and dilute to the mark. Remove any foam rising in the neck of the flask with. a drop of ether... 

Mixing was carried out with a stirrer at 2200 rpm and the formulation was rapidly poured into a large mold provided with thermocouples, located at various heights, to monitor the foam-rising process. 

Average foam rising height, % of the standard sample, at least Mechanical impurities... 

The cell density (number of cells per unit cross-section area or volume) is also used to characterize the coarseness or fineness of foam. Foamed products can feature a deliberately created inhomogeneous (nonuniform) morphology. An example is when a foamed core is sandwiched between solid skins as in so-called structural foams, or in elastomeric products with so-called integral skins. With cells elongated in the direction of foam rise or melt flow, the process will give an anisotropic structure and properties (Chapter 15). 

Measured parallel to foam rise, as maximum stress to 10% deformation. 

The analysis as to whether coalescence or diffusion play a decisive role, using the variation in the distribution function is much more complicated. The curves in Fig. 6.6 show that the polydispersity of the foam rises considerably with time but from the curve type it is not clear which process regulates the course and what is the contribution of each process in the foam collapse. Some indirect data as well as the observations presented in [9], allow to assume that in the initial stage of collapse of polydisperse foams the diffusion transfer is the decisive process, since at the moment of foam formation the films are rather thick and cannot rupture spontaneously. 

Processes of Urethane Foam Preparation. Urethane foams can be prepared by the one-shot process, semi-prepolymer (quasi-prepolymer) process or prepolymer process. The one-shot fnrocess is most commonly used. The semi-prepolymer process is sometimes preferred because of the advantages of easy processing, stabilized-foam rise and lower exotherm. 

In contrast, when polyurethane formation is too fast, the resulting foam cells are stable, cell membranes are not broken during foam rise, and closed cells are formed. The closed cells are the cause of shrunk foams, because the internal gas pressure becomes lower than the atmospheric pressure. 

The NCO/OH equivalent ratio was 5.0 and the aliphaticity index was 0.15. The resultant foam had a cream time of 15 sec, rise time of 20 sec, foam density of 0.041 g/ml, tensile strength in the direction of foam rise of 2.0 kg/cm and a closed-cell content of 90%. 

Frothing was conducted by the co-use of dichlorodifluoromethane, in which the mixing ratio was 2.2 parts of crude MDl per 1.0 part of the solution. The equivalent ratio of NCO/OH was 5.6 and the weight ratio of crude MDI/polyol was 0.14. The frothing mixture was poured into a 100 mm thick panel. The physical properties of the resulting foam were as follows overall density, 0.047 g/cm core density, 0.042 g/cm compressive strength in the direction of foam rise, 2.2 kg/cm the same in the perpendicular direction, 1.6 kg/cm closed-cell percent, 93.8 thermal conductivity, 0.015 kcal/mh C (71). 

One-Step Process. Into 100 g of polymeric isocyanate, 0.8 g of 1-phenyl-3-methyl-l-phospholine oxide, 2.1 g of methanol and 2.1 g of 2,4,6-tris(dimethylaminomethyl) phenol are mixed and stirred for 15 sec. After 10 sec of mixing, foam rise starts, and the rise time is 60 sec. Low-friable foamed products result. 

Reaction Controtters. The cationic polymerization generates high exotherm and results in very fast foam rise, which may cause problems... 

Compressive-strength values are generally of the same order as the tensile strengths in the 2.0- to 3.0-pcf overall-density region. More important is the isotropic nature of the foam, which indicates rounded cells which have almost the same compressive strengths in both parallel and perpendicular directions to foam rise. This is very important in providing dimensionally stable foams. 

In rigid urethane foams, the cell shapes are elliptical like eggs, and, therefore, the compressive strengths in the direction perpendicular to foam rise is smaller than the direction parallel to foam rise. Therefore, if urethane foams are required to have the same compressive strength as pyranyl foams in the direction perpendicular to foam rise (i.e., compressive strength in the direction vertical to the panel substrate) urethane foams must have foam densities greater than those of pyranyl foams. Adhesion of pyranyl foams to various substrates, e.g., steel, phosphated steel, stainless steel and aluminum, as well as paper and wood, is very good. 

Isotropic cell structures, and therefore same mechanical strengths in directions both parallel and perpendicular to foam rise... 

In the prepolymer method for one-shot polyether flexible urethane foams the primary role of the silicone surfactant is to lower surface tension and to provide film (cell-wall) resilience. Resilient films prevent the collapse of the foam during foam rise and continue to stabilize it until the foam is self-supporting. A secondary, but nevertheless important role of the silicone surfactant is cell-size control. The silicones can be added to the formulation in any of the 2 to 6 streams usually fed to the mixing head in the one-shot process. Usually, however, the silicone is metered separately, in combination with the polyol, or added as a wa-ter/amine/silicone mixture. It can also be added in the fluorocarbon blowing agent (52). 

ASTM C 384 is another method used for acoustical materials. This method is an impedance-tube method. In the test a flexible-foam specimen is placed in a cavity over a vacuum chamber. A specified constant-air-pressure differential is then created. The air-flow value is the rate of flow of air required to maintain this pressure differential. The test is carried out 1) with air flow parallel to foam rise, and 2) with air flow perpendicular to foam rise. Air-flow values are proportional to porosity in flexible foams. 

ASTM D 3574 - Test G is another method used for air flow of flexible foams. The test measures the ease with which air passes through a cellular structure. The test consists in placing a flexible-foam core specimen in a cavity over a chamber and creating a specified constant air-pressure differential. The rate of flow of air require to maintain this pressure differential (125 Pa) is the air-flow value. The results are reported in cubic decimeters per second (dm /sec). Tests are usually carried out in two directions parallel to foam rise and perpendicular to foam rise. 

Fibers are frequently used in foam reinforcement. Due to the nature of the process, fibers are oriented as the foam rises (strain and movement) but restricted in movement by bubble wall formation (small distances between bubbles do not leave much freedom for fibers to position themselves). 

Foam Rise Profile. At the end-of-mixing, formulations are poured into a one or two-gallon open-topped container and free rise foaming is measured. The rise profile and rate of foam rise has been measured with a Fluidyne System (4., 5). Foam rise and rate of rise as functions of time after the end-of-mixing are received as graphical output from the Fluidyne System. 

Cell-Opening. In the earlier experiments using a cloud point detector, a surge in carbon dioxide evolution was observed repro-ducibly at a point in the foaming process at which there was a change in shape, an inflexion, in the rate-of-rise curve. In Figure 7, the rate of foam rise is shown as a function of time for Formulation "B." After the maximum rise rate at about 50 seconds, the rate of foam rise decreases rapidly. At 90 seconds, there is an inflexion after which there is a less rapid decline in rise-rate. This point of inflexion corresponds closely to the time the cloud point would be observed. 

TIME, SECONDS, AFTER THE END-OF-M1XING Figure 7. Rate of foam rise, Formulation B. 

Figure 10. Comparison of the rate of foam rise and evolution of carbon dioxide...

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